Monthly values for the Arctic Oscillation Index, which tracks shifts in surface pressure between the mid-latitudes and the Arctic. During the positive phase, the jet stream stays farther north, and mid-latitude winters are milder than usual. Data from NOAA PSL.
Winter surface pressure patterns across the Northern Hemisphere when the Arctic Oscillation (AO) was negative (2009-10) and when it was positive (1988-89). The negative AO favors a warm Arctic and cool conditions across the eastern U.S. Data from NOAA CPC.